Light emitting diodes (LEDs) are different from those conventional light sources in their unique structure and light emitting mechanism, and are more applicable to different industrial fields. For example, LEDs are characterized by their small size, high reliability, and high output, so they are suitable for various kinds of devices, such as indoor or outdoor large displays. Compared to conventional tungsten lamps, LEDs require no filament, consume less power, and respond more quickly, so they are widely used in communication devices or electronic devices. Some main developments of LEDs focus on enhancing the luminance and lowering energy consumption in order to increase competitiveness in the market.
Recently, conductive transparent layer (e.g. ITO, ZnO, etc) are widely used in various photoelectric device, such as TFT-LCD, OLED, and LED, in order to enhance luminance. However, in order to obtain a stable forward voltage while the LED is in operation, it is necessary to form an ohmic contact with the conductive transparent layer. This presents a major problem in the manufacturing of high luminance LEDs
Indium tin oxide (ITO) was a well-known material in forming a conductive transparent layer. Oberman et al, U.S. Pat. No. 5,925,897, proposed a thin composed Au/Ni layer between ITO and the p-type InGaN contact layer. Lin et al, U.S. Pat. No. 6,465,808, proposed a dotted conductive transparent layer to increase light output as the dotted conductive transparent layer has less absorption area. Ludowise et al, U.S. Pat. No. 6,287,947, proposed a multi-layers conductive transparent layer between ITO and the p-type GaN contact layer. The above-mentioned inventions had the disadvantages of high forward voltage because of surface roughness effect during the epitaxy process and hydrogen passivation effect.
The following techniques applied an agent layer between the conductive transparent layer and the LED to lower the forward voltage. Okazaki et al, U.S. Pat. No. 5,977,566, proposed some metal agent layers (including Mg, Ni, Au, Zn, and Ti) to provide an omhic contact. Although the metal agent layer provided the omhic contact with the conductive transparent layer, the metal agent layer, which absorbed light, also decreased the light output. Jou et al, U.S. Pat. No. 6,078,064 proposed a p-type contact layer with high doping concentration (larger than 5*1018 cm−3) as the agent layer, such as GaAs, AlGaAs, ot GaP. Suzuki et al, U.S. Pat. No. 6,479,836, proposed selectively doped super-lattice layers with high p-type carrier concentrations to lower the forward voltage, such as InGaN/GaN, AlGaN/GaN, and etc. Furthermore, the agent layer was provided by transforming Ni into NiO with the oxygen from ITO. Although the agent layer with high doping concentration provides the omhic contact with the conductive transparent layer, it also increases the carrier diffusion effect, and thus decreases the stability and reliability of the devices.
According to the fore-mentioned description, there exists a demand for a high luminance LED with good stability and reliability and operating at low forward voltage.